(CNN) -- A U.S. State Department team arrived in South Korea on Monday in response to a U.S. scientist's report that North Korea has a new uranium enrichment facility.

North Korean officials said the facility is operating and producing low-enriched uranium, according to Stanford University professor Siegfried S. Hecker. The scientist posted a report of his November 12 visit to the Yongbyon, North Korea, facility on the university's website Saturday.

The enrichment facility is comprised of 2,000 centrifuges, according to Hecker's report.

They appear to be designed for nuclear power production, "not to boost North Korea's military capability," Hecker says.

Adm. Mike Mullen, the chairman of the U.S. Joint Chiefs of Staff, told CNN on Sunday that the report "confirms or validates the concern we've had for years about their enriching uranium, which they've denied routinely."

The new nuclear facility violates U.N. Security Council resolutions on North Korea's nuclear program, Mullen said, adding that North Korea is "a country that routinely we are unable to believe that they would do what they say."

Mullen cited the latest report, as well as the sinking of a South Korean ship earlier this year blamed on North Korea, as "belligerent behavior."

"I've been concerned for a long time about instability in that region. Quite frankly, North Korea's been at the center of that," Mullen said. "We've worked hard with other countries to try to bring pressure on them to have them comply. They haven't done that."

On another program, Mullen told the ABC's "This Week" that the new enrichment facility raised the possibility that North Korea would manufacture more nuclear weapons.

"The assumption certainly is that they continue to head in the direction of additional nuclear weapons," Mullen said. "And they also are known to proliferate this technology. So they're a very dangerous country."

Rep. Ileana Ros-Lehtinen, a Florida Republican in line to become chairwoman of the House foreign affairs committee, joined the chorus of those condemning the latest move as "very disturbing."

She said she's urged the State Department's top arms control official to "re-list North Korea as a state sponsor of terrorism," saying it was a "mistake" to remove them from the list "as a reward for cosmetic actions on one component of its nuclear activities." North Korea was taken off the U.S. list of countries that sponsor terrorism in 2008 by the Bush administration, reportedly in a move to encourage North Korean participation in multi-lateral nuclear talks.

"This revelation ... confirms once again the true intentions of the North Korean regime, namely to lull the U.S. and the world to sleep to give it time to advance its clandestine nuclear weapons program," Ros-Lehtinen said in a statement.

Earlier, a senior official in President Barack Obama's administration said that the enrichment program claim is "yet another provocative act of defiance and, if true, contradicts its own pledges and commitments."

The State Department team went to Asia to "begin to coordinate on a response to this news," the official said.

"We have long suspected North Korea of having this kind of capability, and we have regularly raised it with them directly and with our partners in this effort. North Korea has tried to use missiles and nuclear tests to threaten the international community and extract concessions," the administration official said.

Ambassador Stephen Bosworth, who is the Obama administration's point person on North Korea policy, heads the State Department delegation that traveled to Asia. Bosworth met Monday with Wi Sung-lac, South Korea's chief nuclear envoy to the talks on ending North Korea's nuclear ambitions, according to South Korea's Yonhap news agency.

The U.S. delegation's schedule called for travel later Monday to Tokyo, Japan, then on Tuesday to Beijing, China, returning to Washington on Wednesday.

Hecker's report describes the Yongbyon nuclear facilities as "ultra-modern and clean" and notes that this is in sharp contrast to what he witnessed in previous visits.

A Stanford University spokesman told CNN that Hecker was traveling and unable to comment Saturday. The professor plans to speak about his North Korea visit Tuesday at a presentation in Washington, spokesman Michael Freedman said.

In June 2008, North Korea blew up part of its Yongbyon nuclear plant in full view of CNN and a handful of international broadcasters invited to witness the dramatic and symbolic event. It was praised internationally as a move by North Korea away from nuclear weapons capabilities.

The implosion came just 24 hours after Pyongyang had handed in its long-awaited nuclear declaration and after then-U.S. President George W. Bush lifted some sanctions against North Korea and removed it from the U.S. list of states that sponsor terrorism.

Months later, North Korea took a step back, demanding the International Atomic Energy Agency remove surveillance equipment and seals from the Yongbyon nuclear facility. North Korea then reintroduced nuclear material to the facility, and conducted an underground nuclear test in May 2009.

In June 2009, the U.N. Security Council adopted a new sanctions resolution against North Korea over its nuclear program.

Recent satellite imagery taken of North Korea revealed renewed construction at the nuclear power site. Hecker's report confirms the construction is a new nuclear reactor.